A vitreous silica crucible, into which silicon melt is put, is used for pulling silicon single crystal. This vitreous silica crucible is manufactured according to rotational molding by uniformly depositing quartz powder on an inner surface of a rotating mold, and heating and melting the quartz powder for vitrification thereof. In this manufacturing method, it is difficult for the crucible to have a uniform open circumferential portion (rim area) and, therefore, products manufactured by this method need to be finished by cutting the non-uniform rim area. In a related art method, the rim area was manually cut off. However, such a manual cutting of the rim area becomes a burden as the crucible size gets larger, and therefore there has been an attempt to mechanize the cutting process of the crucible for reducing the burden.
As an apparatus for cutting a vitreous silica crucible, a rim cutting apparatus including a supporting means of the crucible and a cutting means of a rim has been known. The supporting means includes an arm which can be opened or closed for gripping the crucible and an operating means thereof for rotational and up-down movement, and the cutting means includes a cutting blade and a moving means thereof. Preferably, the cutting apparatus further includes a mechanism of centering the crucible and a mechanism of taking out the cut rim, and performs a series of operations of centering the crucible, cutting the rim, and taking out the cut rim successively (refer to Japanese Patent Application Laid-Open No. 2001-226134.
The rim cutting apparatus can mechanically cut off a rim of a vitreous silica crucible with high precision, and complete the rim-cutting operation of a large-sized vitreous silica crucible in a short time. Furthermore, since it is possible to automate a series of operations from the centering of the crucible to the taking out of the cut rim, the rim cutting apparatus is considered to be useful for significantly increasing working efficiency.
Meanwhile, when cutting a rim area of a vitreous silica crucible, as illustrated in FIG. 6, a wall is cut by bringing a cutting blade 10 into contact with an outer surface 21 of a sidewall of a crucible 20 and moving the cutting blade 10 toward the inside of the crucible 20, while rotating the crucible 20.
According to the cutting method above, a cutting groove 28 extending around the entire circumference of the crucible is formed in an outer surface 21 of the crucible 20, and the cutting groove 28 progresses in the wall's thickness direction so that a cutting end portion is positioned at an inner surface 22 of the crucible. This way, the load of the cut rim area 25 concentrates on a thin cut section of the inner surface 22 of the crucible 20, so that a crack may be easily generated and a cut edge of the inner surface can be damaged. Thus, burrs 23 and defects 24 may possibly be remain on the inner surface of the crucible after cutting, as illustrated in FIG. 7. In the case where the crucible is cut by bringing the cutting blade 10 into contact with the inner surface of the crucible and moving the cutting blade 10 toward the outer surface, the result is also the same as above.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-226134